The invention relates to a method for the production of silica-based nanocrystallites characterized by a nanoporous (or “mesoporous”) 3-dimensional silica-based network of high water and oxygen permeability, having a granular, monolithic, or hybrid geometric structure. The invention further relates to these nanocrystallites, which, inter alia, are highly translucent with excellent optical clarity, and exceptionally high porosity, low density, and high permeability characteristics and which have superior thermal insulating, vibration (dynamic), and acoustic (static) barrier properties.
Silica-based hydrogels such as aerogels, xerogels, nanogels, ambigels, and the hydrogels of the present invention are chemically inert, highly porous ceramic materials useful in many applications. They are commonly produced by sol-gel processes based on hydrolysis/polycondensation (H/P) reactions well-known in the art. Typically, a sol is prepared from a source of silica such as a silicate or alkoxide by dispersing the silica source in a synthesis solvent comprising water or a water/alkanol solution and one or more gelation catalysts. Products of silica hydrolysis then condense, forming a sol system comprising a network of linked silica particles. Often, a silating agent is introduced to cap free hydroxyl groups on the polycondensation products, thereby rendering them hydrophobic.
After the sol system reaches its gel point, the sol-gel is set aside to age, allowing hydrolysis and condensation of reactants to continue while the sol-gel self-assembles, strengthening the gel structure and increasing its density; this step also influences the optical, mechanical, acoustic, thermal and other properties of the gel. During the aging step, the gel is usually contacted with pure alcohol or other low surface tension topping agent to displace water of condensation present in the nanoporous structure of the gel. The product wet hydrogel is then dried.
Drying the wet hydrogel, which at this stage is weakly structured, is a critical step in this process. Owing to the high capillary stress exerted on the wet gel network during drying, this intermediate product is at high risk of compression, extended cracking, shrinkage, and pore collapse, particularly in highly porous, low density gel structures having relatively high surface areas, e.g., of at least about 600 m2/g, such as aerogels in the range of about 700 m2/g and the hydrogels of this invention (also referred to herein as “CrystalGel hydrogel”), whose surface areas are in the range of about 1000 m2/g. If care is not taken in the drying process, the wet gels are prone to structural weakness and significant brittleness (friability) when dried.
To avoid this outcome, many gel drying processes have been proposed in this art. Alkane drying solvents have been used for ambigels and xerogels, which have surface areas in the ranges of about 600 m2/g and 400 m2/g, respectively. However, this is a costly and a tedious process which requires numerous washes, extensive pollution abatement steps, and expensive hazard prevention equipment, although gel porosity may often be well-preserved. Alternate methods for drying these gels include the use of high-purity acetone or a similar extraction solvent for water removal, followed by acetone replacement with a solvent of low surface tension such as high-purity hexane, heptane, or octane, which minimizes stresses caused by otherwise rapid evaporation of the extraction solvent. The low surface tension solvent is then removed, as by decanting, and any residual solvent is allowed to slowly evaporate or is removed under vacuum to preserve porosity. Aerogels, of higher porosity, higher surface area, and lower density than the ambigels and xerogels, are commonly now dried at supercritical temperatures and pressures in the range of about 95-104° F. and about 1200-1500 psi under CO2 in autoclaves or comparable high pressure apparatus. Porosity retention values of the supercritically dried products can range up to about 95%; however, the drying equipment is expensive and the drying conditions are often very hazardous.
It is accordingly desirable to provide methods for the preparation of highly porous and highly permeable silica-based hydrogels which include a drying step that minimizes internal stresses on the wet gel structure during drying, obviating cracking, shrinkage, pore loss, and other detrimental effects on the product dried gel, and which does not require the use of toxic solvents or extreme process conditions. It is further desirable to provide a silica-based crystallite of high porosity, high-permeability, and low density with excellent translucency, clarity, thermal insulation, vibration and acoustic barrier, and bulk modulus properties as producible by this process.